1. Field of the Invention
The present invention relates to boosting circuits, particularly to a boosting circuit formed of a plurality of boosting circuit units.
2. Description of the Background Art
Many conventional boosting circuits are configured so that the capacitive element included in each of a plurality of boosting circuit units is connected in parallel. However, a boosting circuit of such a configuration has the boosting efficiency degraded by various factors such as the fabrication condition, the back gate effect, and the like of the transistor generally included as a circuit element of a boosting circuit unit.
A boosting circuit that suppresses such reduction in boosting efficiency is disclosed in, for example, Japanese Patent Laying-Open No. 63-316510. The boosting circuit disclosed in this publication is configured by having a plurality of boosting circuit units (boosting circuit blocks) connected in series.
Each boosting circuit block is operated by a clock input. The clock input turns on/off a plurality of transistors included in each boosting circuit block, whereby the capacitive element (capacitor for boosting) included in each of the plurality of boosting circuit blocks is switched between a charging state and a series connection state. The turn on/off of a plurality of transistors included in each boosting circuit block in response to a clock input is called “activation of a boosting circuit block” hereinafter.
Since the capacitor for boosting included in each of a plurality of boosting circuit blocks is connected in series in the boosting circuit disclosed in Japanese Patent Laying-Open No. 63-316510, the influence of the transistor fabrication condition, back gate effect and the like are eliminated to suppress reduction in the boosting efficiency.
It is to be noted that the boosting circuit disclosed in Japanese Patent Laying-Open No. 63-316510 has the clock input in common to all the boosting circuit blocks, as shown in FIG. 1 in this publication. This means that all the boosting circuit blocks are rendered active at the same time.
When the capacitor for boosting included in each of the plurality of boosting circuit blocks attains a series connection state, charge is supplied to all the capacitors for boosting starting from the input of the boosting circuit block of the first stage. Therefore, the current from the input power supply will congregate at the circuit element connected closest to the input of the first boosting circuit block. Power supply noise will be generated by this current congregation from the input power source to a particular circuit element, resulting in the problem of degradation in the boosting efficiency of the boosting circuit.
Also, further current congregation from the input power source to a particular circuit element will cause electromigration, whereby the boosting efficiency of the boosting circuit will be further degraded.